1. Field of the Invention
The present invention relates to a method for manufacturing an SOI substrate having a silicon thin film on a transparent insulative substrate.
2. Description of the Related Art
An SOI (Silicon on Insulator) wafer having a single-crystal silicon (Si) layer on an insulator film such as an oxide film is used as a device forming substrate for the purpose of forming an MOS transistor which is superior in radiation-proof characteristics or latch-up characteristics and also superior in suppression of a short-channel effect. Among others, a low-defect SOI substrate obtained by applying a bonding technology attracts attention. Further, in recent years, using such an SOI substrate as an optical device manufacturing substrate has been examined (e.g., Japanese Patent Application Laid-open No. 145438-1999, Japanese Patent Application Laid-open No. 18926-1994, Japanese Patent Application Laid-open No. 163363-1999, Japanese Patent Application Laid-open No. 2003-282885 and Japanese Patent Application Laid-open No. 2002-110998).
In regard to an SOI substrate having a silicon thin film on an insulator substrate, an SOITEC method (a SmartCut method) of bonding a silicon substrate having hydrogen ions implanted on a bonding surface side to a handling substrate (a support substrate) and performing a heat treatment at approximately 500° C. or above to thermally delaminate the silicon thin film from a region having the highest concentration of the implanted hydrogen ions is known. This method is based on a mechanism of adopting heating to “grow” high-density “air bubbles” called “hydrogen blisters” formed by implantation of the hydrogen ions and utilizing this “air bubble growth” to delaminate the silicon thin film (e.g., Japanese Patent No. 3048201 or A. J. Auberton-Herve et al., “SMART CUT TECHNOLOGY: INDUSTRIAL STATUS of SOI WAFER PRODUCTION and NEW MATERIAL DEVELOPMENTS” (Electrochemical Society Proceedings Volume 99-3 (1999) p. 93-106)).
Meanwhile, a single-crystal silicon substrate is roughly classified into an FZ silicon substrate grown by a floating zone (FZ) method and a CZ silicon substrate grown by a Czochralski (CZ) method, and the CZ silicon substrate occupies 80% or more of a total amount used. This CZ silicon substrate is manufactured by heating polysilicon put in a crucible formed of high-purity quartz by a graphite heater to be dissolved, growing an ingot having a large diameter by a technique of gradually increasing a crystal diameter of a narrow seed crystal, and slicing and polishing this grown ingot. However, when the dissolved silicon reacts with the quartz crucible at a high temperature during ingot growth, an oxygen impurity, which has eluted in a silicon solution, is introduced into the crystal at a concentration of approximately 20 to 30 ppma. The thus introduced oxygen impurity is called “interstitial oxygen” since it is placed between lattices of a silicon crystal. This interstitial oxygen is precipitated in the crystal since it enters a supersaturation state during an ingot cooling process or a device manufacturing process, and it becomes an electrically active “defect” depending on its precipitation conformation.
For example, in a case where a heat treatment at approximately 500° C. or above is required in a process for manufacturing an SOI substrate like the above-explained SOITEC method (the SmartCut method), a so-called “new donor” is produced when this heat treatment is carried out in a temperature range of 600 to 700° C., and a so-called “thermal donor” is produced when this heat treatment is performed in a temperature range of 400 to 500° C. These defects (oxygen donors) results in a problem that electrical characteristics (a resistivity) of a silicon layer of an obtained SOI substrate become different from those at the initial stage.
In view of the above-explained problem, it is an object of the present invention to provide an SOI substrate having no worry about a fluctuation in electrical characteristics due to generation of oxygen donors.